Address Resolution Protocol (hereinafter referred to as ARP) is stipulated under RFC 826, by David C. Plummer (November 1982). As such, the ARP constitutes a standard in the Transmission Control Protocol/Internet Protocol (TCP/IP). Specifically, the ARP is a protocol used by a source host to acquire a physical address of a destination host. A general description is provided in a book by Douglas E. Comer entitled "Internetworking With TCP/IP Principles, Protocols, and Architecture", published by Prentice-Hall, Inc. (1988). While the internet protocol (IP) address may be known, the media access control (MAC) address may not be known. The MAC address is the specific address of a host.
The format of packets used under the ARP will now be described with reference to FIG. 1. FIG. 1 shows the format of packets used for the Ethernet (registered trademark of Fuji Xerox Co., Ltd.). The Ethernet packet format comprises a destination physical address 1, a source physical address 2, an Ethernet type (value: 0806H; where H stands for hexadecimal) 3, an ARP message 4 and an error correction code 5.
FIG. 2 depicts the format of the ARP message 4, wherein a hardware type field 6 designates the type of hardware interface from which the source host requests an acknowledgment. For the Ethernet, the hardware type is represented by the value 1. Likewise, a protocol type field 7 specifies the type of high level protocol address used by the source host. For an IP address, the protocol type is represented by the value 0800H. An operation field 8 indicates an ARP request (value: 1) or an ARP acknowledgment (value: 2). An HLEN field 9 and a PLEN field 10 designate a high level protocol address length (4 bytes for the IP) and a physical address length (6 bytes for the Ethernet), respectively. A sender HA field 11 and a sender IP field 12 denote the physical address and IP address of the source host, respectively. A target HA field 13 and a target IP field 14 specify the physical address and IP address of the destination host, respectively. The target HA field 13 is to be set by the destination host upon ARP acknowledgment.
The operation of the ARP will now be described with reference to FIGS. 3A and 3B. Reference characters A15, B16, X17 and Y18 denote hosts. Each host is connected to the same physical segment on the Ethernet 19. When the host A15 (IP address: Ia; physical address: Pa) is to obtain the physical address of the host B16 (IP address: Ib; physical address: Pb), the host A15 broadcasts an ARP request packet of FIG. 4 (in the manner shown in FIG. 3A). The destination physical address "all FFH" in the ARP request packet of FIG. 4 indicates that the packet is to be broadcast. In so doing, the host A15 requests the host B16 to return its physical address Pb. Although all hosts including the host B16 receive the same request, only the host B16 recognizes the IP address in the requesting packet. In response, the host B16 sends to the host A15 an ARP reply packet including its own physical address Pb (see FIG. 3B). This is how the host A15 acquires the physical address of the host B16.
Of course, it is possible that all of the hosts place ARP request packets onto the network whenever necessary, thereby increasing the loads on the network. Such events are circumvented by a scheme under which the ARP request packet broadcast by, for example, host A15 is received by the other hosts caching the IP address Ia and physical address Pa of the host A15.